1. Field of the Invention
This invention relates to a roller frame, including at least two rollers mounted in a housing, between which a nip can be formed. The ends of the rollers are supported in rotary fashion in bearings and at least one of the rollers an advancing roller and has an adjusting system by which the bearings of the advancing roller can be moved, thus changing the nip.
2. Discussion of Related Art
Roller frames of this kind are used, for example, in the manufacture of plastic sheets and plates. A typical example of use are so-called calender stacks in which a molten mass produced in an extrusion process, after emerging from a sheet die, is guided through an adjustable calender nip between two rollers. The molten mass is cooled in the process and shaped in such a way that a sheet web is produced with a uniform thickness and a homogeneous appearance across the entire sheet surface. So-called calendered sheets of this kind are made, for example, of polystyrene, polypropylene, or polyester and are used among, other things for transporting and protecting foods and are also used in the automotive industry.
Generally, a calender stack includes at least two rollers, one of which is stationary and at least one other roller is embodied as an advancing roller. This advancing roller can be moved by an adjusting system so that different calender nip settings can be produced between the stationary roller and the adjustable roller. In this way, it is possible to produce sheets of different thicknesses.
Known calender stacks are usually equipped with a hydraulic adjusting system, which has various disadvantages. The hydraulic fluid used in them, typically oil, contaminates the sheet webs when leaks occur, thus rendering them unusable. Also, the use of hydraulic fluid that must be regularly replaced is expensive and the corresponding apparatus takes up a relatively large amount of space. Finally, hydraulic systems frequently cannot be controlled with the required degree of precision so that additional measuring devices must be used for determining the calender tip that has been set and for readjusting it as needed.
U.S. Pat. No. 4,414,713 A discloses a calender with a plurality of rollers in which the pressing force of the top roller on the other rollers situated below it can be produced by an adjusting mechanism composed of a rack-and-pinion system. The nip is adjusted by the bottom roller, which is acted on by hydraulic, cylinders.
U.S. Pat. No. 2,861,504 A and U.S. Pat. No. 1,989,038 A describe calenders with an adjusting device for the rollers that acts by a spindle/spindle nut system. Systems of this kind, with a spindle and a spindle nut traveling on it, however, have an inevitably large amount of play in both adjustment directions in order to ensure adjustability, which makes such designs unsuitable for precise nip adjustment, such as in a calender stack.